Planar optical waveguides are an attractive tool for use in analytical chemistry and spectroscopy. A wide variety of inorganic and organic materials have been used to fabricate thin-film waveguides, and as a result, planar guides can be engineered for specific chemical applications. As the evanescent wave is easily accessed, a number of papers have addressed the use of planar waveguides for bio/chemical sensors. Attenuation, fluorescence, and interferometric sensors have been reported, as has the use of waveguides for enhanced Raman spectroscopy.
Unlike fiber optics, planar waveguides have been slow to be widely accepted due to the difficulty of coupling light into the waveguide. In the laboratory, prism coupling is the predominant method, followed by end fire and grating coupling. Prism coupling, which operates on the principle of frustrated total internal reflectance, and endfire coupling, which uses fiber optics or a lens to introduce light directly into the polished endface of the waveguide, are highly efficient methods, as typically 80% of the laser beam is coupled into the waveguide. The use of prisms and fibers does not damage the waveguide, and the various elements (prisms, fibers, and lenses) are reusable. They are impractical for routine use, however, as both coupling methods require expensive positioning equipment. Prism coupling is sensitive to environmental fluctuations and destroys the two-dimensional geometry of the planar waveguide. Diffraction or reflection gratings for light coupling into planar waveguides are more practical than prisms or fibers for routine use. Although the coupling efficiency observed with gratings is reduced, the two-dimensional nature of the guide is conserved and gratings are generally more robust than prisms. Furthermore, the coupled power is immune to environmental fluctuations because the grating is often embedded in the waveguide.
Grating couplers are commonly fabricated using techniques based on holography. This approach involves an exposure step using a single mirror which creates an interference pattern between two spatial halves of a laser beam. The exposed photoresist acts as a mask for chemical etching of the underlying waveguide or substrate to form a periodic grating structure. This process can be time consuming, since this method involves an exposure followed by a chemical etch. Blazed gratings require additional fabrication steps. The use of an embossing technique where the surface relief pattern of a master grating is pressed in to a suitable material may provide a fast and economical method to form grating couplers for routine use.
Several investigators have published methods to emboss gratings for waveguide applications. The earliest was Wei et al. (Wei, J. S.; Tan, C. C. "Coupling to film waveguides with reusable plastic gratings", Appl. Op., 1976, 15, 289.) who used a thick (&gt;100 .mu.m) film of a polycarbonate that was poured onto a master grating. The polycarbonate film was subsequently peeled from the master grating and "stuck" on the waveguide surface. Although this method is easy, it is not amenable to mass production. Furthermore, reduced efficiency is observed due to the use of an extremely thick polycarbonate film and poor contact between the grating and the waveguide surface.
This was followed by the work of Lukosz (Lukosz, W.; Tiefenthaler, K. "Embossing technique for fabricating integrated optical components in hard inorganic waveguiding materials", Opt. lett. 1983, 8, 537-539) who embossed gratings into sol-gel glasses. Although this technique uses a master grating to impress a replica into a thin film guide, it is limited to sol-gel glass type waveguides. Furthermore, subsequent work (Roncone, R. L.; Weller-Brophy, L. A.; Weisenbach, L.; Zelinski, B. J. J. "Embossing gratings in sol-gel waveguides: pre=emboss heat treatment effects", J. Non.Cryst. Solids 1991, 128, 111-117) showed that the grating pattern was not uniformly transferred and that blaze (grating profiles) was distorted.
Christensen and Dyer (Christensen, D.; Dyer, S.; Herron, J.; Hlady, V. "Comparison of robust coupling techniques for planar waveguide immunosensors", Proc. SPIE, 1992, 1796, pp. 20-25) improved the embossing technique by coating the master grating with a vacuum deposited aluminum film. The grating pattern is replicated onto the waveguide surface with a UV curable epoxy. Because the aluminum film does not adhere strongly to the grating, it "releases" the master from the cured epoxy replicate. This type of grating replication technique can be applied to all waveguide types. The limitation however is that the master grating is not truly reusable, for each new grating embossed the aluminum release film must be reapplied to the master grating. Mass production, and the resulting economies of scale, are thus impossible.